Field
The disclosure relates to a light emitting device, particularly to a light emitting device with a lens which is arranged to cover a light emitting element.
Description of the Related Art
Light emitting devices that use semiconductor chips (light emitting elements) such as light emitting diodes can be easily reduced in size while obtaining high luminous efficiency and are used widely. Among those, some light emitting devices have, for example, a lens (i.e. a sealing portion which serves as a lens) to refract light from the semiconductor chip in a desired direction, in order to obtain a larger amount of light in a specific direction such as in the upward direction of the substrate. The light emitting devices of this type equipped with a lens are adapted to many applications such as lighting and backlights.
The lens used in such a light emitting device is generally disposed on a substrate so as to cover the light emitting element. A significant amount of light emitted from the light emitting element is refracted by the lens and directed from the upper surface of the lens toward the desired direction. However, with such an arrangement of the lens, most portion of the lower surface of the lens may be in contact with the surface of the substrate. As a result, a part of light emitted from the light emitting element, particularly the light in a downward direction exits the lens and reaches the substrate. This is due to a difference between the refractive index n1 of the light-transmissive material such as glass or a resin which constitutes the lens and the refractive index n2 of the material which constitutes the mounting substrate. That is, in the case where n2 is larger than n1, total reflection does not occur, or in the case where n1 is larger than n2 but the difference between n1 and n2 is too small, the critical angle for total reflection increases.
Accordingly, a part of light reaching the substrate is reflected and returned inside the lens, but significant part of light reaching the substrate is absorbed by the substrate. The absorption of light by the substrate can be reduced by increasing the reflectance of the surface of the substrate. However, even with such a known technique to reduce the absorption, significant amount of the light is absorbed by the substrate, which may result in a decrease of the light extraction efficiency. For this reason, for example, as shown in JP 2010-251666A, there has been known a light emitting device in which a lens (particularly the bottom surface of the lens) is extended to outside of the substrate in a plan view. FIG. 15 is a cross-sectional view schematically showing a cross-section of a known light emitting device 500 in which the bottom surface of the lens is extended to outside of the substrate.
In the light emitting device 500, a light emitting element 10 is mounted on the substrate 4. The light emitting element 10 and the upper surface of the substrate 4 are arranged within the lens 502, and a portion of the substrate 4 is exposed in the bottom surface of the lens 502. Also, the bottom surface of the lens 502 is extended to outside of the substrate 4 in a top view along a direction perpendicular to the upper surface of the substrate 4 (the −Z direction in FIG. 15).
As shown in FIG. 15, of the bottom surface of the lens 502, the portion which is extended to the outside of the substrate 4 is in contact with the air. The air has a small refractive index, so that the difference between the refractive index n1 of the light transmissive material such as glass or a resin which constituting the lens and the refractive index nair of the air can be large and which leads a small value of the critical angle for total reflection. As a result, most of the light reaching the bottom surface of the lens 502 is reflected and thus the amount of light exiting to outside from the bottom surface of the lens 502 can be reduced. With this arrangement, the light emitting device 500 can provide high extraction efficiency, particularly high extraction efficiency from the upper surface of the lens.
In recent years, a light emitting device which has much higher light extraction efficiency, particularly at the upper surface side of the lens than that of a conventional light emitting device has been demanded, and for this, a light emitting device which allows for a further reduction of leakage of light to the mounting surface side (i.e., downward with respect to the bottom surface of the lens) has been demanded.